Since the thickness of the mobile information terminal such as a smartphone is reduced, or a camera mounted on the mobile information terminal is getting higher pixels, an imaging lens of the camera is required to be thin and large-caliber.
For example, Patent Document 1 disclosed an imaging system composed of five lenses, so as to reduce the thickness, to achieve a brightness under F2.5 and a wide angle, and to correct aberrations.
Additionally, Patent Document 2 disclosed an imaging system composed of six lenses, so as to correct the aberrations and to get high resolution while achieving a wide angle with a full field angle larger than 80°.                Patent Document 1: Japan Patent Publication No. 2016-018001.        Patent Document 2: Japan Patent Publication No. 2015-007748.        
However, a large-caliber imaging lens needs to achieve a brightness under F2.2, or further under F2.0. As a result, the imaging lenses disclosed in Patent Document 1 and Patent Document 2 are not sufficiently thin.